1. Field of the Invention
The present invention relates to a system and method for monitoring a high-frequency circuit that operates at low temperatures to handle electrical signals in the spectral range of quasi-microwaves, microwaves, or millimeter waves. More particularly, the invention pertains to a monitoring system, as well as to a monitoring method therefor, that observes variations in the frequency response of a high-frequency circuit with minimum insertion loss.
2. Description of the Related Art
Recent years have seen an increased demand for high-quality mobile network systems and satellite communications systems to meet the needs for wideband data transport of videos and images with better quality. To achieve the purpose, those communications systems use high frequency bands such as quasi-microwaves, microwaves, or millimeter waves. As one of the constituent technologies, low-loss high-frequency components (e.g., communications filter products) with small size and light weight will certainly play an essential role in the system development. Those circuits should handle high-frequency signals in the spectral range mentioned above. One requirement in this aspect is that the communications system has to incorporate some kind of monitoring and correction mechanism, so that the system will be able to check itself as to whether each circuit has an intended frequency response, and correct itself if necessary.
High-frequency circuits used in such a communications system include passive components using oxide superconductors designed for operation at cryogenic temperatures as low as several tens of kelvins (K). Think of, for example, a high-frequency analog and/or digital circuit that operates at 90 K or below. The following shows several methods and techniques used in observing the frequency response of this kind of circuit.
(1) The frequency response of a circuit of interest is directly measured in an experimental setup with a signal generator and a spectrum analyzer. Specifically, directional couplers, isolators, power distributors, and other necessary instruments are connected to the input and output of the circuit under test in a way suitable for each specific circuit configuration. The frequency response is identified by sweeping the output frequency of the signal generator within an intended frequency range while making the spectrum analyzer track that frequency sweep.
(2) The frequency response is measured in a similar way, but using a network analyzer in which both a signal oscillator and spectrum analyzer are integrated.
(3) In the case the circuit of interest has no particular inputs, its output signal is observed with a spectrum analyzer. For this purpose, a directional coupler or signal distributor is used to split a part of the output signal.
(4) Instead of using a spectrum analyzer, a sampling oscilloscope is attached to the circuit to observe its output in the time domain. This method is applicable if the frequency range is ten-odd gigahertz or below.
(5) Instead of using an analog signal generator, a digital signal generator is attached to the circuit. This configuration is applied when the circuit of interest handles digital input signals.
(6) The output of the high-frequency circuit is observed through a directional coupler, signal distributor, or other necessary circuit.
(7) A test signal is entered to the circuit through a directional coupler or other appropriate component placed at the input port of the circuit.
Typically, in any of the above cases (1) to (7), the high-frequency circuit of interest is located in a cryostat for operation in a low temperature environment. On the other hand, the attachments (e.g., couplers and distributors) are placed outside the cryostat, meaning that they are set in an environment at room temperature or near room temperature.
As an example of a passive circuit using oxide superconductive material, let us consider a planar circuit (e.g., microstrip lines, coplanar circuit) with a copper-oxide superconductive film formed on a substrate. This type of construct is used in high-frequency filters, for instance, and copper-oxide high-temperature superconductors are suitable material for the film because they are known to have a good crystallinity and show less energy loss (or high Q) in quasi-microwave and microwave applications, compared to ordinary materials including copper, silver, gold, aluminum, or others that exhibit high electrical conductivity. Further, the circuit may be placed in an ultra-low temperature environment to increase the conductivity, while there are some problems that have to be solved before it is put into practical use. That is, theoretically, copper-oxide high-temperature superconductors are expected to show a better performance than ordinary conductors in millimeter band and above (i.e., 0.3 THz and higher in the frequency domain) if it is cooled down to near the liquid helium (LHe) temperature, which is 4.2 K.
In the above section, we have discussed high-frequency circuits that handle electrical signals with quasi-microwave, microwave, or millimeter wave components, operate at cryogenic temperatures under 100 K, and have a transmission line to carry a signal over a conductor where electromagnetic fields concentrate. The frequency response of such circuits can be monitored by using the techniques (1) to (7) described above. They are, however, for use in laboratory-level systems. While a packaged high-frequency circuit can fit in a space of several to several hundred cubic centimeters, the entire system including circuit analysis devices is as large as several to several hundred liters typically. However, most part of this space requirement is attributed to, for example, the display of a spectrum analyzer which shows the result of measurement. The size of the system can therefore be reduced if it is allowed to limit what and how to observe for frequency response measurement.
Another important aspect of the monitoring system is the transmission loss that a high-frequency signal will experience when it passes through some additional circuits attached for the purpose of monitoring. While the amount of loss may depend on what frequency range is used or how the circuit is configured, the presence of loss often becomes a real problem when the quality of signals is critical. For example, a typical transmission line circuit made of ordinary conductors, together with its accompanying high-frequency connection medium such as coaxial cables, will reduce the signal level by a few tenths to several decibels at both input and output ends of a high-frequency circuit being monitored. In the case of superconductor-based digital circuits using Josephson junctions, the insertion loss of additional circuits eventually reduces their fan-outs. Also, analog circuits handling small signals would encounter a problem of low input levels when insertion loss is present. In high-power analog circuits, the monitoring circuits waste their output power. Further, a directional coupler with a large coupling factor often causes a problem of distortion or loss of input and output signals of the high-frequency circuit to which the coupler is attached.